The present invention is related to an improved method of forming cathode materials and an improved capacitor formed thereby. More specifically, the present invention is related to an improved capacitor with improved reliability.
The construction and manufacture of solid electrolytic capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal preferably serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal is electrolytically formed to cover all surfaces of the anode and serves as the dielectric of the capacitor. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide or electrically conductive organic materials, such as polyaniline, polypyrrole, polyethylenedioxythiophene and their derivatives. Solid electrolytic capacitors with intrinsically conductive polymers as the solid cathode electrolyte material have been widely used in the electronics industry due to their advantageous low equivalent series resistance (ESR) and “non-burning/non-ignition” failure mode. In the case of conductive polymer cathodes, the conductive polymer is typically applied by chemical oxidation polymerization, electrochemical oxidation polymerization, prepolymerized dispersion, etc. with other less desirable techniques being reported. U.S. Pat. No. 7,563,290, which is incorporated herein by reference, describes the polymer slurry or conducting polymer dispersion process. The resulted capacitors show excellent high voltage performances, reduced DC leakage (DCL) and improved long term reliability. However, the conductive polymer dispersions have relatively high moisture sorption in comparison with in-situ polymerized conductive polymer. U.S. Pat. No. 8,902,567, which is incorporated herein by reference, describes various methods to manage moisture in solid electrolytic capacitors.
In addition to the conducting polymer solid electrolyte, the cathodic layer of a solid electrolyte capacitor typically consists of several layers, which are external to the solid cathode electrolyte. These extended layers typically include a carbon containing layer and a layer comprising a conductive metal either formed by electrodeposition or the metal is bound in polymer or resin matrix such as in a metal filled adhesive. The metal layer is then attached to a cathode lead by a conductive adhesive. The layers including the solid cathode electrolyte, conductive adhesive and layers there between are referred to collectively herein as the cathode layer which typically includes multiple layers designed to allow adhesion on one face to the dielectric and on the other face to the cathode lead. A highly conductive metal lead frame is used as a cathode lead for negative termination. The various layers connect the solid electrolyte to the outside circuit and also serve to protect the dielectric from thermo-mechanical damage that may occur during subsequent processing, board mounting, or customer use.
The conductive particles, such as the carbon or metal, in the cathode layers are typically held together by binders including monomeric, oligomeric or polymeric materials or resins. The binder also helps to form adhesive bonds between the highly conductive cathode layer and the lead frame. On exposure to humidity, moisture sorbed by the binder, or by the conductive particles themselves in the case of conductive polymer dispersions, tends to swell the layers causing an increase in ESR due to either delamination or an increase in the distance between conductive particles. Another weakness of the binder is that they tend to degrade at high temperatures and high humidity, which affects the cathode connection integrity. One method to improve the integrity of these binders is to use thermoset binders instead of thermoplastic binders. Thermoset resin based encapsulants are used in solid electrolytic capacitors for their thermal, chemical, and environmental stability. However the high crosslinking density of unfilled thermoset resins makes them inherently brittle. This brittleness of thermoset resin is one of the major obstacles preventing their wider use in cathode layers in solid electrolytic capacitors. To overcome the lack of toughness, thermoplastic polymers have been incorporated into thermoset resins for toughening.
U.S. Pat. No. 8,767,378, which is incorporated herein by reference, claims an electrically conductive paste composition comprising binder resins comprising aliphatic thermoplastic resin and self-polymerizing thermosetting resin. WO Patent application No. 2013111438, which is incorporated herein by reference, claims a conductive paste where the resin group consist of butyral resins, acrylic resins, epoxy resins, phenoxy resins, phenol resins, amino resins, and urethane resins. U.S. Pat. Application No. 20150009606, which is incorporated herein by reference, claims the electrode layer is formed from a conductive paste that includes at least a conductive filler, a thermosetting resin containing a thermoplastic resin such as phenoxy resin, and a curing agent.
Studies suggest that increasing the degree of crosslinking decreases ESR shift when capacitors are exposed to high humidity conditions. However, heavily crosslinked thermoset coatings are more susceptible to cracking under thermo-mechanical stress such as those stresses caused by surface mount technology (SMT) and subsequent operational thermal shock. Another disadvantage of thermoset systems is that they are typically low molecular weight and are low viscosity materials. The low MW/low viscosity binder properties lead to faster settling of conductive particles. Thermoplastics with high molecular weight and high viscosity binders, such as polyacrylic, polyester, polyvinyl butyral, phenoxy, polyurethane are known in the art as additives to the thermoset polymers, such as phenolic, and although this improves the coverage and processability, humidity caused ESR shift will increase due to a lower degree of crosslinking and thus a faster diffusion of moisture into the cathode.
However, the use of thermoplastic polymers in thermoset resins reduces moisture barrier properties, stiffness, strength, and creep resistance of the toughened polymer system. Thus, a need exists for an improved method of toughening thermoset polymers in cathode layers without affecting the moisture barrier, humidity, thermal, and other environmental properties. An improved capacitor, and method of making the capacitor, are provided herein.